How actin goes in new directions

By Rajendrani Mukhopadhyay

The cytoskeleton protein actin plays a critical role in cell movement by assembling at cell protrusions. The assembly process involves actin-binding proteins, one of which is the actin-related protein 2/3, or Arp2/3, complex. This complex helps to form branched actin structures, which allow actin to push the membrane envelope forward and get the cell to migrate. But how Arp2/3 picks out specific actin networks is not well understood. In a recent “Paper of the Week” in The Journal of Biological Chemistry, a team led by C.-L. Albert Wang at the Boston Biomedical Research Institute demonstrated that caldesmon, an actin-binding protein, increased the Arp2/3-mediated branching activity at newly formed actin filaments. By using in vitro and imaging assays, Wang and colleagues found that caldesmon had no effect on branch formation at older actin filaments, but the younger, fresher actin filaments were twice as active in forming branches than the mature actin filaments. This suggested that caldesmon maintains freshly polymerized actin in a state with a higher affinity for the Arp2/3 complex. Wang explains that the group is now working to determine if other actin-binding proteins have the same effect as caldesmon in modulating actin cytoskeletal dynamics and if caldesmon can affect cell migration and tumor metastasis by manipulating actin dynamics.